A. Field of Invention
This invention relates to the magnetic recording art and in particular to a novel and improved method for preparing an alignment disc pak.
Disc paks generally comprise one or more magnetic discs upon which digital information can be recorded and played back by means of a disc drive which includes motive means for rotating the disc, an arm which carries transducers for recording and playing back the information and associated electronics. Information is recorded on the disc in concentric tracks with a typical track density of 100 or 200 tracks per inch dependent upon the disc drive application. The transducer arm is indexable to any one of the recording tracks, there being 202 tracks in one typical disc used in a number of commercial disc drives. Many computer installations include several disc drives to provide additional memory to the internal memory of a central processing unit. It is absolutely essential in such installations that a disc pak recorded on any one of the disc drives be capable of record and/or playback operation on any of the other disc drives without error. This requirement dictates that the transducer arms of all the disc drives be in alignment with one another within certain tolerance limits. Al alignment disc pak is used to accomplish this alignment of the transducer arms of a number of disc drives.
B. Prior Art
Prior Art alignment disc paks have been prepared by recording periodic signals of different frequencies f1 and f2 either in adjacent tracks or in adjacent bands which straddle one of the tracks. Although these signals could be recorded at any one of the tracks, they have been typically recorded at track no 73, where track numbering begins with zero for the outermost track and proceeds inwardly to tract no. 202 for the track nearest the geometric center of the disc. These signals are recorded on the disc with an eccentric spindle, i.e., a spindle that is offset from the normal geometric center by a small distance (typically on the order of 0.0015 in.). when such a disc is played back on a disc drive with a normal or noneccentric spindle, the transducers will pick up the signal of frequency f1 for a portion of a disc revolution and the signal of frequency f2 for the remainder of the disc revolution due to the eccentricity of the band in which the signals are recorded. This playback signal includes signal components of frequency f1 and frequency f2 and also a further signal component of frequency (f1-f2), where f1 is greater than f2. When the playback signal is displayed on an oscilloscope, the (f1-f2) signal component appears as an envelope in a horizontal figure eight pattern, commonly called a "cat's eye" pattern, i.e., one having two outer crossovers and an intermediate crossover.
To align the transducer arm of a disc drive with the alignment disc pak, the transducer arm is indexed to track 73 (or to whichever track the aligmment signals are recorded) and the playback signal is coupled to an oscilloscope to display the cat's eye pattern. If there is equal horizontal distance (time base) between the intermediate crossover and each of the outer crossovers, the transducer arm is in alignment, meaning that the transducer picks up each recorded signal for substantially one-half of a disc revolution. If the horizontal distance between the intermediate crossover and the outer crossover are unequal, the transducer arm is manually adjusted until they are so equal. As this equality comparison is a visual process, equality can be achieved only within tolerance limits and, of course, varies from one human observer to another. The problem is compounded because the envelope appears as a rather thick bright trace consisting of a series of short, closely spaced, vertical lines on a background of a lighter trace of the other two signal components. This makes it extremely difficult to visually observe time base equality of the cat's eye lobes, particularly in disc drive applications requiring a track density of 200 tracks per inch.